Bacterial community structure and metabolomic profiles of yak milk and cattle-yak milk during refrigeration in Gannan region: Analysis of interspecific differences in milk spoilage

The bacterial community dynamics and metabolomic profiles in raw yak (Y) milk and cattle-yak (CY) milk during refrigeration at 4 °C were investigated, followed by the elucidation of interspecific differences in milk storage. Bacterial communities and succession patterns were significantly different between the two milk types during refrigeration, with Lactococcus and Pseudomonas being the key distinguishing genera. Moreover, higher network complexity and tighter interactions were observed for the microbial community in CY milk than in Y milk. Furthermore, 7 proteases and 1 lipase potentially contributed to milk spoilage. The metabolomic profiles significantly differed between the milk types during refrigeration. Extended storage time decreased the relative abundances of organic nitrogen compounds and lipids and lipid-like molecules, with a concomitant increase in organic acids and derivatives, particularly in Y milk. Moreover, 9 metabolites, whose levels gradually increased with storage time, were strongly correlated with psychrophiles and thus considered potential markers of deterioration in plateau-characteristic milk. These findings offer a theoretical foundation for augmenting the quality and safety of plateau-characteristic milk and its derivatives, while also helping us understand the microbial and metabolic dynamics in raw milk under extreme environments.